A team led by
biologists at the University of California, San Diego has discovered
a molecule in roundworms that makes them susceptible to Bacillus
thuringiensis toxin, or Bt toxin—a pesticide
produced by bacteria and widely used by organic farmers and
in genetically engineered crops to ward off insect pests.

Their findings should
facilitate the design and use of Bt toxins to prevent
insects, which the researchers believe also possess the molecule,
from developing resistance to Bt, extending the life
of this natural pesticide.

The study, published
February 11 in the journal Science, details the structure
of a molecule to which Bt attaches, or “binds,”
in the lining of the intestines of insects and roundworms. The
molecule is a glycolipid—a lipid attached to a tree-like
arrangement of sugars. Because changes in the sugars impact
Bt’s ability to bind, the researchers believe
that their discovery will make it possible to develop better
pesticides and lead to new treatments for parasitic infections
that affect close to two billion people worldwide.

Image
of normal roundworm with graphic of Bt toxin binding
to molecule in intestine (top) and resistant roundworm in
which Bt cannot bind.Credit: Joel Griffitts, Stanford University

“Our previous
findings with the roundworm C. elegans strongly suggested
that specific sugar structures are likely critical for Bt
toxin susceptibility,” said Joel Griffitts, the first
author on the paper and a former graduate student with UCSD
biology professor Raffi Aroian. “This latest paper demonstrates
what these sugars actually do. They provide a receptor for the
toxin that allows the toxin to recognize its “victim”—a
roundworm or an insect. This paper also brings us from the conceptual
realm to the chemical nature of these sugar structures—how
their atoms are arranged, and how the toxin binds to them.”

“Bt
toxin, which is produced by a soil bacterium, is toxic to insects
and roundworms, but not to vertebrates, which accounts for its
popularity as a pesticide,” explained Aroian, who led
the team. “But the development of insect resistance to
Bt is a major threat to its long term use. Our findings
make it possible to understand resistance at the molecular level
and should improve resistance management.”

In collaboration with
Paul Cremer and Tinglu Yang, coauthors on the paper and chemists
at Texas A&M University, Griffitts and Aroian found that
Bt toxin directly binds glycolipids. However, in each
of the four Bt resistant mutants tested—bre-2,
bre-3, bre-4 and bre-5—the researchers found
that there was either zero or dramatically reduced binding of
glycolipids to Bt toxin. They concluded that the defective
sugar structure of the glycolipid receptor in each of the mutants
prevents Bt from binding.

Other members of the
research team, coauthors Stuart Haslam and Anne Dell, biologists
at Imperial College London; Barbara Mulloy, a biochemist at
the Laboratory for Molecular Structure, National Institute for
Biological Standards and Control in Hertfordshire, England;
and Howard Morris, a biochemist at the M-SCAN Mass Spectrometry
Research and Training Centre in Berkshire England, determined
the chemical structure of the normal glycolipid receptor that
binds Bt toxin.

Elements of this structure
are found in both insects and nematodes, but are not found in
vertebrates at all, which may be one reason these proteins are
safe to vertebrates. This work furthermore opens up the possibility
of using Bt toxins against roundworms that parasitize
humans.

“These parasites
infect nearly one-third of the human population and pose a significant
health problem in developing countries,” said Aroian.
“Perhaps one-day vertebrate-safe Bt toxins could
be used as human therapies against these parasites.”

Griffitts and Aroian
credit the flexibility of the roundworm C. elegans
as an experimental system, particularly the ease of manipulating
it genetically, in making it possible to find and characterize
the structure of the long sought-after Bt receptor.
However, their results apply to insects as well. Michael Adang
and Stephan Garczynski, coauthors and entomologists at the University
of Georgia, showed that the glycolipid receptor is present in
the tobacco hornworm, an insect pest that is susceptible to
Bt toxins used commercially in plants.

“It will now
be possible to monitor insect populations near fields where
Bt is used and catch insect resistance in its early
stages by looking for changes in glycolipids,” said Aroian.
“If changes are detected, switching to another pesticide,
perhaps even another variety of Bt that works through
a different mechanism, could prevent the resistance genes from
becoming widespread.”

According to the researchers,
prior work indicates that there are other receptors that also
contribute to Bt resistance. Combining pesticides that
work through different receptors or designing pesticides that
can work through more than one receptor type could thwart the
development of resistance.

“This paper presents
an intriguing question,” said Griffitts. “In light
of findings by insect biologists that certain proteins
function as important Bt toxin receptors in some cases,
how might glycolipid and protein receptors cooperate to engage
this intoxication program? If the field can figure this out,
it might allow for the engineering of toxins that can utilize
either type of receptor alternatively, such that host resistance
would require the mutation of both receptor types. This means
that resistance would be exponentially less probable.”

The study was funded
by the National Science Foundation, the Burroughs-Wellcome Foundation
and the Beckman Foundation.